A galvanic isolator provides a way for transmitting a signal from one electrical circuit to another electrical circuit in a control system when the two electrical circuits may otherwise be electrically isolated from one another. Usually the two electrical circuits operate at different voltages, and thus, are electrically isolated. For example, consider an application in which a 5V battery powered controller board is configured to control a motor circuit operating at 240V. In this example, the 240V motor circuit may be electrically isolated from the 5V controller circuit, while permitting the 5V controller circuit to send or receive signals from the 240V motor circuit. In another example involving a solid-state lighting system, a 240V Alternate Current (AC) power supply may be converted to two different Direct Current (DC) power domains. The two DC power domains are electrically isolated as there is no direct current path between the two DC domains, but there may be control signals that need to be communicated between the two power domains. In these applications, an isolator may be used to provide voltage and/or noise isolation while still permitting signaling and/or information exchange between the two circuit systems.
Galvanic isolators may be further categorized into opto-isolators, capacitive isolators, magnetic isolators and radio frequency based isolators depending on the technology used to electrically isolate the electrical circuits from one another. An opto-isolator may comprise an optical emitter and an optical receiver. Over time, degradation may occur and optical signals emitted from the optical emitter may degrade. Opto-isolators are usually for low frequency applications because photodiode as well as light-emitting diodes used as emitter in most capacitive isolators have built-in-capacitance that limits the transmission speed of opto-isolators.
Capacitive isolators may not have the optical degradation issue of the opto-isolators. However, incorporating high voltage capacitor into a semiconductor die may be technically challenging. Capacitors that are fabricated by using conventional semiconductor process may not meet the requirement of high voltage tolerance. For example, opto-isolators may be able to meet isolation requirement such as breakdown voltage specification of 8 kV. However, typically most capacitive isolators fabricated under conventional CMOS process have breakdown voltage of 2 kV, which is relatively low compared to opto-isolators.
Most capacitive isolators available today have off-chip capacitors relying on capacitors outside a semiconductor package. Some of these capacitive isolators may have capacitors arranged in series in order to meet the breakdown voltage specification. However, having capacitors in series also means floating electrical node that is not testable, and may be susceptible to noise.